Silicon, while being an excellent material in many respects of importance for the microelectronics industry as well as for the fabrication of passive optical devices, suffers from one major drawback; the inability to sustain efficient stimulated light emission by electrical pumping. In effect, this means that a laser that can be made to lase by introducing current through it made purely out of silicon is not feasible today. Since lasers are the fundamental light sources for all optical data transfer systems, this is a severe shortcoming. Up to now, no fully satisfactory solution to this problem has been presented.
Compound semiconductors, such as e.g. III-V semiconductors having direct bandgap, can however be utilized for achieving stimulated light emission. Despite their superior electronic properties with respect to silicon, cost aspects have favored silicon for microelectronics. Attempts to combine compound semiconductor laser components with silicon or SiO2 substrates or waveguides have been performed. The closest such attempt is based on what is called bonding technology in which a layer of a compound semiconductor is transferred either to a silicon substrate, or directly to a Si/SiO2 waveguide. Different variations on this approach exist, such as bonding an entire compound semiconductor substrate to a silicon substrate, bonding a pre-fabricated compound semiconductor photonic device die to a silicon substrate or bonding a stack of compound semiconductor material, on which an active photonic device can be subsequently fabricated, to a silicon substrate. Of these approaches, the latter seems to be the most promising, for the reason that it allows flexibility in alignment and enjoying advantages of economics to scale since dies can be bonded to a substrate of any size, unlike a substrate which must match the size of the host substrate for scale economics to be advantageous.
In the published US patent applications 2007/0170417 A1 and 2009/0245298 A1, photonic integrated circuits on silicon are disclosed. By bonding a wafer of III-V material as an active region to silicon and removing the substrate, the lasers, amplifiers, modulators and other devices can be processed using standard photolithographic techniques on the silicon substrate.
Indeed, active photonics devices have been successfully fabricated based on these approaches, such as light sources, detectors and modulators. However, some problems still persist. The first is related to both approaches where dies (normally bonded by adhesive bonding in which a polymer is used as adhesive) and where substrates (normally achieved by direct bonding in which two substrates have a common SiO2 interface without an adhesive) are bonded. Since the vast majority of the several hundred μm-thick compound semiconductor substrate goes to waste. This is undesirable since compound semiconductor substrates, such as InP or GaAs which are the most widely used for fabrication of lasers emitting at telecom wavelengths, are expensive. In addition, they are only available in sizes much smaller than that of silicon. This means that fabrication of active devices cannot benefit from advantages associated with economics of scale by moving to larger substrates as demand increases. An additional drawback in the case of adhesive bonding pertains to the properties of the bonding medium; so far, the most promising results have been obtained with the polymer known as Benzocyclobutene (BCB). Whereas this material possesses desirable properties in terms of low optical loss, good adhesion at low temperatures and the ability to planarize surfaces, it has poor thermal conductivity. Since heat dissipation is a major issue in active devices such as lasers, this is a serious drawback that must be remedied by subsequent formations of thermal vias through which the heat can escape. This is not as big an issue in the case of direct bonding, although SiO2 also has relatively poor heat conduction. An additional problem with direct bonding is however void formation at the SiO2 interface.
Finally, there are some issues with the bonding step itself in terms of yield and efficacy; firstly, there exists as of yet no process for bonding dies to a wafer in a production volume-friendly way, and secondly, although alignment is not critical if the bonded dies do not contain pre-fabricated lasers, the accuracy in alignment that is practically achievable puts a limit on the density of useful devices that can be achieved.
In the published US patent application 2007/0170417 A1, an electrically pumped semiconductor evanescent laser is disclosed. An optical waveguide is disposed in silicon. An active semiconductor material is disposed over the optical waveguide defining en evanescent coupling interface between the optical waveguide and the active semiconductor material.
To our knowledge prior art attempts to grow active semiconductor material directly on a Si/SiO2 waveguide have not been undertaken, although growth of active semiconductor material on silicon has been tried extensively. The probable reason is that in epitaxial growth of a compound semiconductor directly on silicon, defects will result. Among these defects are so-called threading dislocations, spontaneously forming to release the strain that arises since compound semiconductors such as GaAs and InP have lattice constants as well as coefficients of thermal expansion that differ from that of silicon. These defects degrade device performance, in particular by radically decreasing device lifetime.